1. Field of the Invention:
The present invention relates to a method of detecting the concentration of oxygen in a gas to be measured, and it also relates to a method of controlling the air-to-fuel ratio in an engine based on the detected oxygen concentration. More specifically, the invention relates to a method of detecting the concentration of oxygen in a gas to be measured using a limiting current type oxygen sensor, and a method of controlling the air-to-fuel ratio in an engine based on the detected oxygen concentration.
2. Description of the Prior Art
Limiting current type oxygen sensors have been conventionally employed for detecting the concentration of oxygen in a gas to be measured. According to the limiting current type oxygen sensors, an oxygen sensor element body is made of an oxygen-ion permeable solid electrolyte with electrode plates being provided on the opposite surfaces of the element body, and with one of the electrode plates (a negative electrode) coated with a porous ceramic layer thicker than that covering the other (a positive electrode).
When in use, the oxygen sensor having the oxygen sensor element thus constituted therein is mounted on a specific place in such a manner that the oxygen sensor element may be brought into contact with a gas to be measured. The application of voltage to the electrode plates causes a limiting current to flow through the oxygen sensor element depending upon the concentration of oxygen in the gas. Thus, the intensity of this current outputted from the oxygen sensor element is measured to detect the concentration of oxygen in the gas to be measured.
The conventional method for detecting the oxygen concentration using such a limiting current oxygen sensor will be described below with reference to FIG. 1.
FIG. 1 is a graph illustrating the relationship between the voltage applied to the oxygen sensor and the current outputted from it. The curves 1, 2 and 3 represent the voltage-electric current characteristics respectively when the concentrations of oxygen in the gases to be measured are 1%, 5%, 10%.
When a voltage is applied to the opposite electrode plates of the limiting current type oxygen sensor element, oxygen in the gas to be measured is ionized at the negative electrode to permeate through the oxygen sensor element toward the positive electrode. With the increase in the voltage applied to the oxygen sensor element, the output electric current increases correspondingly or proportionally. In FIG. 1, reference numerals 1a, 2a, 3a represent first stage leading edges (portions) of the curves 1, 2, 3 in the respective oxygen concentrations. When the voltage applied to the oxygen sensor element exceeds a certain value, the amount of the permeating oxygen from the negative electrode to positive electrode is limited because the negative electrode of the sensor element is covered with the porous ceramic layer. Thus, when the applied voltage is further increased, the current obtained is kept constant, i.e., the so-called limiting current is outputted. Reference numerals 1b, 2b, 3b represent the flat portions of the characteristic curves 1, 2, 3 at the different oxygen concentrations respectively. The values of the limiting current proportionally vary with the oxygen concentrations, and further the range of the voltage which give rise to a limiting current depends upon the oxygen concentration. When the voltage applied becomes larger than this range producing the limiting current, the outputted electric current begins to increase again with the applied voltage. Reference numerals 1c, 2c, 3c represent the second stage leading edges (portions) of the characteristic curves at the different oxygen concentrations respectively.
Assume that a voltage falling within the voltage range which gives rise to the current intensities at the flat portions 1b, 2b and 3b at all the different oxygen concentrations respectively is selected and that such a voltage is applied to the oxygen sensor element. Then, the oxygen concentration in a gas to be detected can be determined by the measurement of the outputted current because the oxygen sensor element outputs the limiting current in proportion to the oxygen concentration. However, the voltage ranges giving the flat portions at the different oxygen concentrations of the characteristic curves shift little by little.
For this reason, when a certain amplitude of voltage shown at the vertical line "B" in FIG. 1 is applied to the opposite electrodes of the limiting current type oxygen sensor, and the oxygen concentration is kept, for instance, at 1%, 5% or 10%, the current with the intensity at an ordinate of the intersecting point between the line "B" and the characteristic curve 1, 2 or 3 is outputted from the oxygen sensor. The relationship between the outputted electric current and the different oxygen concentrations is preliminarily determined. Then, the concentration of oxygen contained in a gas to be measured is detected with reference to the preliminarily determined relationship between the outputted current and the oxygen concentration. According to the conventional method, however, since the voltage applied to the oxygen sensor element is set constant, it is not necessarily possible to detect with accuracy the concentration of oxygen, because in some cases, the limit current corresponding to the specific oxygen concentration cannot be outputted.
For instance, when the voltage applied is set at the constant value "x.sub.1 " as shown in the vertical line "B", if the oxygen concentration is 5% or 10% (the characteristic curves 2 and 3), the oxygen sensor outputs an accurate limiting current intensities y.sub.1 and y.sub.2 at these oxygen concentrations because the line "B" intersects the characteristic curve 2 or 3 at flat portion 2b, or 3b i.e., the applied voltage falls within the specified ranges giving rise to flat portions 2b and 3b. On the other hand, when the oxygen concentration is 1%, the vertical line "B" intersects the curved portion, or the second stage leading edge (portion) 1c, not at the flat portion 1b. Consequently, the intensity of the electric current outputted from the oxygen sensor becomes larger than that of the limiting current y.sub.12. Thus, the oxygen sensor gives an inaccurate measurement of the oxygen concentration in the gas to be measured.
FIG. 2 illustrates in a one dotted chain line "B.sub.1 " the relationship between the electric current outputted from the oxygen sensor and the oxygen concentration when the voltage applied to the sensor element is set constant as shown by "B" of FIG. 1. The one dotted chain line "B.sub.1 " corresponds to the state when the line "B" is shifted to the right as shown in FIG. 1. To the contrary, the two dotted chain line B.sub.2 corresponds to the state that the line B is shifted to the left different from the line B in FIG. 1.
The outputted electric current is converted into the oxygen concentration based on this curve "B(B.sub.1 or B.sub.2)". From FIG. 2, it is seen that the lineality of the outputted current-oxygen concentration conversion line "B" is shifted upwardly at the low concentration, while the lineality at more than 10% of the oxygen concentration is shifted downwardly in the line "B.sub.2 " because the flat portion of the corresponding characteristic curve is deviated to the right in FIG. 1 and accordingly, the current outputted at the applied voltage x becomes higher or lower than the limiting current.
In this way, since the range in the oxygen concentration which produces their limiting currents becomes restricted when the voltage applied to the oxygen sensor is set constant, the conventional method cannot give accurate measurement of the oxygen concentrations when they vary over a wide range.
In addition, the ranges in the voltage which give rise to the flat portions of the limiting current in the characteristic curves may differ from one another due to the variance in the characteristics of the different oxygen sensors. The above-described conventional method cannot satisfactorily dispose of such a variance.
Another problem is the difficulty of obtaining satisfactory control of the air-to-fuel ratio in such a system where the outputted electric current proportional to the oxygen concentration in a gas to be measured is detected using the limiting current type oxygen sensor, and where a control signal based on this outputted current is inputted to the fuel supply means to control the air-to-fuel ratio in the engine.
In order to eliminate the problems encountered by the conventional method, it is considered preferable to widen the flat portions of the characteristic curves. However, the present level of technology makes this difficult.